1. Field of the Invention
The present invention relates to semiconductor integrated circuits and, more particularly, to a current mirror for providing a large current ratio and a high output impedance and a differential amplifier having the same.
2. Description of the Related Art
A current mirror is a circuit for mirroring an amount of current flow of one circuit branch into another circuit branch. The current mirror serves as a constant current source for supplying a predetermined amount of current regardless of load and is generally used to set a DC bias of a circuit.
FIG. 1 is a circuit diagram of a conventional current mirror. Referring to FIG. 1, an output current Iout, for example, a mirrored current, is obtained by multiplying the ratio of a size A2 of an output transistor 13 to a size A1 of a reference transistor 12 with a reference current Iref. However, to reduce current consumption in a radio frequency (RF) circuit, in general, current smaller than 100 xcexcA is used as the reference current Iref, the output current Iout must be greater than 1 mA to be used as a bias current. Thus, in a case where the size A1 of the reference transistor 12 is 1, the size A2 of the output transistor 13 must be greater than 10.
In this case, an output impedance of the current mirror in low frequency operation relates only to the amount of the output current Iout (Ve/Iout) (Ve is early voltage), and does not relate to the size A2 of the output transistor 13. However, parasitic components, for example, parasitic capacitance between a base and a collector of the output transistor 13 and parasitic capacitance between a substrate and a collector, occur in radio frequency operation, thereby lowering the output impedance of the current mirror. And the parasitic components are increased as the size A2 of the output transistor 13 increases.
A current mirror is generally used as a current source of a differential amplifier. If an output impedance of the current mirror decreases, a common mode rejection ratio (CMRR) of the differential amplifier is lowered, and thus precise differential signals are not generated. Further, a differential amplifier in which one terminal is grounded, is used as an active balun in a local oscillator (LO) of a RF mixer. However, the CMRR of the differential amplifier used in the LO of the RF mixer is low and precise differential signals are not generated, the characteristics of the mixer are greatly lowered. Accordingly, a need exists for a current mirror for providing a large current ratio and a high output impedance even in radio frequency operation and a differential amplifier for generating precise differential signals without lowering a common mode rejection ratio (CMRR).
A current mirror is provided, which includes: a current source for supplying a reference current, the current source having a first terminal connected to a first reference voltage and a second terminal; a reference transistor having a control electrode, a first current carrying electrode connected to the second terminal of the current source, and a second current carrying electrode connected to a second reference voltage; an output transistor having a control electrode, a first current carrying electrode connected to an output terminal to which a mirrored current flows, and a second current carrying electrode connected to the second reference voltage; and a proportional-to-absolute temperature (PTAT) voltage generator connected between the control electrode of the reference transistor and the control electrode of the output transistor, for increasing the voltage of the control electrode of the reference transistor to a predetermined voltage and supplying the increased voltage to the control electrode of the output transistor. The first reference voltage is a supply voltage, and the second reference voltage is a ground voltage.
According to an embodiment of the present invention, the current mirror further includes: a first resistor connected between the second current carrying electrode of the reference transistor and the second reference voltage; and a second resistor connected between the second current carrying electrode of the output transistor and the second reference voltage. The first reference voltage is a supply voltage, and the second reference voltage is a ground voltage. The predetermined voltage is obtained by             V      t        xc3x97    ln    ⁢          xe2x80x83        ⁢          (                        Z          xc3x97                      A            1                                    A          2                    )        ,
wherein Vt is a thermal voltage, Z is the resistance ratio of the first resistor to the second resistor, and A1 and A2 are the sizes of emitters of the reference transistor and the output transistor, respectively.
According to an embodiment of the present invention, the PTAT voltage generator includes: a first transistor having a first current carrying electrode connected to the second terminal of the current source, a control electrode connected to the first current carrying electrode, and a second current carrying electrode; a second transistor having a control electrode connected to the control electrode of the first transistor, a first current carrying electrode connected to the first reference voltage, and a second current carrying electrode; a third transistor having a control electrode connected to the second current carrying electrode of the second transistor, a first current carrying electrode connected to the second current carrying electrode of the first transistor, and a second current carrying electrode connected to the first current carrying electrode of the reference transistor; and a fourth transistor having a control electrode connected to the second current carrying electrode of the first transistor, a first current carrying electrode connected to the second current carrying electrode of the second transistor, and a second current carrying electrode connected to the control electrode of the output transistor. The first through fourth transistors of the PTAT voltage generator are bipolar transistors.
According to an embodiment of the present invention, the PTAT voltage generator includes: a first transistor having a first current carrying electrode connected to the first reference voltage, a control electrode, and a second current carrying electrode; a second transistor having a control electrode connected to the control electrode of the first transistor, a first current carrying electrode connected to the first reference voltage, and a second current carrying electrode connected to its control electrode; a third transistor having a control electrode connected to the second current carrying electrode of the first transistor, a first current carrying electrode connected to the second current carrying electrode of the first transistor, and a second current carrying electrode connected to the first current carrying electrode of the reference transistor; and a fourth transistor having a control electrode connected to the control electrode of the third transistor, a first current carrying electrode connected to the second current carrying electrode of the second transistor, and a second current carrying electrode connected to the control electrode of the output transistor. The first and second transistors are MOS transistors, and the third and fourth transistors are bipolar transistors.
According to an embodiment of the present invention, the reference transistor and the output transistor are bipolar transistors.
A differential amplifier is also provided, which includes: a differential amplifying unit for differentially amplifying two input signals; and a current mirror for generating a mirrored current proportional to a predetermined reference current and supplying the mirrored current as a current source to the differential amplifying unit, wherein the current mirror includes: a current source for supplying a reference current, the current source having a first terminal connected to a first reference voltage and a second terminal; a reference transistor having a control electrode, a first current carrying electrode connected to the second terminal of the current source, and a second current carrying electrode connected to a second reference voltage; an output transistor having a control electrode, a first current carrying electrode connected to an output terminal to which a mirrored current flows, and a second current carrying electrode connected to the second reference voltage; and a proportional-to-absolute temperature (PTAT) voltage generator connected between the control electrode of the reference transistor and the control electrode of the output transistor, for increasing the voltage of the control electrode of the reference transistor to a predetermined voltage and supplying the increased voltage to the control electrode of the output transistor.
According to an embodiment of the present invention, the current mirror includes: a first resistor, the first resister having a first terminal connected to the second current carrying electrode of the reference transistor and a second terminal connected to the second reference voltage; and a second resistor, the second resister having a first terminal connected to the second current carrying electrode of the output transistor and a second terminal connected to the second reference voltage. The first reference voltage is a supply voltage, and the second reference voltage is a ground voltage. The predetermined voltage is obtained by             V      t        xc3x97    ln    ⁢          xe2x80x83        ⁢          (                        Z          xc3x97                      A            1                                    A          2                    )        ,
wherein Vt is a thermal voltage, Z is the resistance ratio of the first resistor to the second resistor, and A1 and A2 are the sizes of emitters of the reference transistor and the output transistor, respectively.
According to an embodiment of the present invention, the PTAT voltage generator includes: a first transistor having a first current carrying electrode connected to the second terminal of the current source, a control electrode connected to the first current carrying electrode, and a second current carrying electrode; a second transistor having a control electrode connected to the control electrode of the first transistor, a first current carrying electrode connected to the first reference voltage, and a second current carrying electrode; a third transistor having a control electrode connected to the second current carrying electrode of the second transistor, a first current carrying electrode connected to the second current carrying electrode of the first transistor, and a second current carrying electrode connected to the first current carrying electrode of the reference transistor; and a fourth transistor having a control electrode connected to the second current carrying electrode of the first transistor, a first current carrying electrode connected to the second current carrying electrode of the second transistor, and a second current carrying electrode connected to the control electrode of the output transistor. The first through fourth transistors are bipolar transistors.
According to an embodiment of the present invention, the PTAT voltage generator includes: a first transistor having a first current carrying electrode connected to the first reference voltage, a control electrode, and a second current carrying electrode; a second transistor having a control electrode connected to the control electrode of the first transistor, a first current carrying electrode connected to the first reference voltage, and a second current carrying electrode connected to its control electrode; a third transistor having a control electrode connected to the second current carrying electrode of the first transistor, a first current carrying electrode connected to the second current carrying electrode of the first transistor, and a second current carrying electrode connected to the first current carrying electrode of the reference transistor; and a fourth transistor having a control electrode connected to the control electrode of the third transistor, a first current carrying electrode connected to the second current carrying electrode of the second transistor, and a second current carrying electrode connected to the control electrode of the output transistor.
A method for generating a mirrored current is also provided, which includes the steps of: supplying a reference current to a collector of a bipolar reference transistor; increasing a base voltage of the bipolar reference transistor to a predetermined voltage to supply a current having a predetermined proportion to the reference current to generate the mirrored current; and supplying the increased voltage to a base of a bipolar output transistor.
According to an embodiment of the present invention, the predetermined portion is a ratio in size of a plurality of transistors connected to the reference transistor versus a plurality of transistors connected to the output transistor.